Low-cost fiber optic pressure sensor

ABSTRACT

The size and cost of fabricating fiber optic pressure sensors is reduced by fabricating the membrane of the sensor in a non-planar shape. The design of the sensors may be made in such a way that the non-planar membrane becomes a part of an air-tight cavity, so as to make the membrane resilient due to the air-cushion effect of the air-tight cavity. Such non-planar membranes are easier to make and attach.

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and the University of California for the operation of LawrenceLivermore National Laboratory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates fiber optic pressure sensors, and morespecifically, it relates to methods for reducing the size of fiber opticpressure sensors.

2. Description of Related Art

Optical fibers have been used widely for sensing various physical orchemical entities. In many applications optical fiber is used because itis very thin, being typically 125 μm (0.125 mm). One example is anengine cylinder pressure sensor to be embedded in a spark plug. In suchan application, the diameter of the whole sensor should be less than 1mm. A typical sensor for such an application consists of a tubing with aplanar membrane attached at the end and an optical fiber insertedinside. A light from an optical fiber impinges on the membrane and apart of the light is reflected (either by a simple reflection or by aninterferometric effect) from the membrane and is coupled back into thefiber and/or other fibers. One major fabrication difficulty in thissensor type is the attachment of the membrane to the end of the smalltubing. When the tubing size becomes very small such an operationbecomes impossible. Even when attachment can be done, the productioncost becomes higher as the size gets smaller. These problems havehindered the wider application of optical fiber sensors.

Among many advantages of using optical fibers for sensors, one distinctadvantage is the thinness of its diameter. An optical fiber is typicallymade of fused quartz, and its typical outer diameter (OD) is 125 μm(=0.125 mm). The OD may be reduced even further to 60 μm or even to 20μm since the light-guiding core is 50 μm for the commonly-used multimodefiber, and less than 9 μm for the most commonly-used fibers, namelysingle-mode fibers. Accordingly, there are a variety of applicationswhere a long and thin fiber strand is inserted into a location that isotherwise inaccessible, to measure some physical or chemical entity. Onesensor configuration is depicted in a highly schematic fashion in FIG.1, in which an optical fiber 1 is inserted into a thin tubing 2 that isterminated by a planar membrane 3. FIG. 2 shows the sectional view ofthe embodiment of FIG. 1. Even though FIG. 1 shows only one strand ofoptical fiber, it is to be understood throughout this inventiondisclosure that there can be more than one fiber involved, such as abundle of fibers. This understanding does not alter the validity andgenerality of the present invention.

The sensing mechanisms can vary. In some cases, the membrane 3 may beexposed to a certain gas or liquid that produces a fluorescence lightemission. The fiber 1 would collect the emission and the wavelengthcontent of the collected light provides information on the nature of thegas or liquid: identification, concentration and/or temperature. Inother cases, the membrane may be deflected by an external stimuli, suchas acoustic wave, gas pressure, liquid pressure, or physical pressure.The deflection may be detected by first sending light through the fiber1 and then directing the light returning through the fiber or fibers 1onto a detector. One common physical mechanism to induce change in theamount of the returned light is Fabry-Perot interferometry. The other isby amplitude modulation. The present invention works with any of thesemechanisms, and thus the particularly of the sensing mechanism orsensing entities is not the subject of this invention. With this pointunderstood, the present invention will be described using one simplesensing mechanism, namely the amplitude modulation. FIG. 3 shows a fiber1 with the core 5 terminated near the membrane 3 (FIG. 3 is actually aclose-up view of FIG. 2 near the fiber end). The light impinges on themembrane 3 and is reflected. In this process, only a part of the light 6is coupled back to the core 5. There could be fibers, other than theinput fiber 1, for collecting the reflected light so that the fiber 1 isused only for transmitting the light 6 to the membrane 3. This is forinstrumentation convenience. Now, referring to FIG. 4, if the membrane 3is deflected, the pattern of the light reflection is altered, and theamount of the light 6 being coupled back into the fiber 1 changes. Theamount of the change indicates the amount of the deflection. (In aninterferometric sensor, the change in the gap modulates the lightreflection; a change by one-quarter wavelength causes a full swingbetween the maximum and the minimum in the reflected light power. Inthis case, only one fiber 1 is used for sending and collecting the light6.)

The market size of fiber optic sensors is very large, exceeding a fewhundreds of millions of dollars today, and it is still expandingrapidly. One technical stumbling block is the fabrication. When the sizeof the tubing becomes smaller than about 1 mm, it becomes more difficultto attach the membrane 3 at the end of the tubing 2. And there are manyapplications in which small size is essential.

SUMMARY OF THE INVENTION

It is an object of the invention to make to reduce the size of fiberoptic pressure sensors.

It is another object of the invention to reduce the cost of fabricatingsmall-size fiber optic pressure sensors.

The difficulty in reducing the size of fiber optic pressure sensors ismainly related to the requirement that the membrane should be smooth andtaut. The smoothness is required for satisfactory light reflection, andthe tautness is required for reproducibility, quick response, and alsosatisfactory reflection. This difficulty can be lessened substantiallyif the membrane is designed to have a non-planar surface in its naturalstate (in the absence of a stimulus such as a pressure), such asspherical, conical or wedge shape. Then some of the conventionalfabrication methods such as extrusion and forming can be mobilized muchmore readily for the sensor fabrication.

It may be necessary in some applications (when the membrane material istoo thin or too flexible) to transform the area under a non-planarmembrane into a cavity using a plugging material, so as to make theenclosed space air-tight. Then the non-planar membrane can keep itsshape and remain taut like a balloon surface as the surface becomesresilient due to the air-cushion effect of the air-tight cavity.

As an example of fabrication methods, one can literally blow a balloon,in which a molten or liquid state material is blown by a pressureapplied to the other end of the tubing until it forms a semi-sphere. Theextra amount of the molten or liquid state material will be blown out ofthe tubing, leaving behind a thin balloon-like membrane. The material 12is then solidified, either (i) thermally using a heater, (ii) as themolten material cools down, or (iii) by polymerization by ultra-violet(UV) exposure if it is a UV-curable polymer. Liquid rubber materialcalled RTV, which solidifies by itself over a time period, is anotherexcellent raw material for making the novel membrane. A lowmelting-temperature glass may be blown into the sphere inside ahigh-melting temperature tubing such as quartz or tungsten. Thespherical nature of the surface does not have to be pronounced. So longas the surface is even slightly convex, it will be much easier tofabricate the membrane following the teaching described here.

Another embodiment with a great potential uses a small ball that is madeseparately and then inserted inside the tubing, or attached to thetubing. There are conventional fabrication methods for making miniaturesize spheres or balls, and fiber optic sensors can utilize such spheres.The surface of the ball that is exposed outside the tubing will work asthe membrane. In many applications, the membrane should be thin.However, there are other applications in which the stimulus is verypowerful. An example is the engine cylinder pressure sensor. Thepressure can reach about 1,000 psi. In such applications the ball 16should be solid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a conventional fiber optic sensor comprisingan optical fiber, a tubing, and a planar membrane.

FIG. 2 shows a sectional view of the sensor in FIG. 1.

FIG. 3 shows a close-up view of the sensor of FIG. 2, depicting thatlight from the fiber end facet is reflected from the membrane.

FIG. 4 shows a close-up view of the sensor of FIG. 2, except that themembrane is deflected by an external stimulus.

FIG. 5 shows one embodiment of the present invention, in which themembrane has a spherical shape.

FIG. 6 shows another embodiment of the present invention, where the baseof the spherical membrane is confined within the inner diameter of thetubing.

FIG. 7 shows another embodiment of the present invention, in which themembrane has a conical or wedge shape.

FIG. 8 shows another embodiment of the invention having an inner spaceof the tubing near the spherical membrane that is sealed to make thespace air-tight.

FIGS. 9A-C illustrate one simple method for making the sphericalmembrane, in which (9A) a liquid or molten material is blown to asemisphere (9B), and then solidified (9C).

FIG. 10 shows the surface of the ball exposed outside the tubing for useas the membrane.

FIG. 11 shows the ball of FIG. 10 as a solid.

DETAILED DESCRIPTION OF THE INVENTION

The objective of the present invention is to make it possible to reducethe size beyond what is possible today, and also reduce the cost offabricating small-size fiber optic sensors. The difficulty is mainlyrelated to the requirement that the membrane 3 should be smooth andtaut. The smoothness is required for satisfactory light reflection, andthe tautness is required for reproducibility, quick response, and alsosatisfactory reflection. When the tubing size becomes very small, themembrane thickness should become thinner as well, so that theflexibility of the membrane remains the same. When the tubing sizeshrinks below 1 mm, it becomes more expensive or even impossible to makea thin membrane, cut into the right size, and attach it to the end ofthe tubing while keeping it smooth and taut. This difficulty can belessened substantially if we allow the membrane to have a non-planarsurface in its natural state (in the absence of a stimulus such as apressure), such as spherical 7 and 8, conical or wedge shape 9, asdepicted in FIGS. 5 through 7. Then some of the conventional fabricationmethods such as extrusion and forming can be mobilized much more readilyfor the sensor fabrication.

It may be necessary in some applications (when the membrane material istoo thin or too flexible) to transform the area under a non-planarmembrane, 7, 8, or 9, into a cavity using a plugging material 10, so asto make the enclosed space air-tight, as depicted in FIG. 8. FIG. 8shows the use of a spherical membrane. Then the non-planar membrane cankeep its shape and remain taut like a balloon surface as the surfacebecomes resilient due to the air-cushion effect of the air-tight cavity.

As an example of fabrication methods, one can literally blow a balloon,as depicted in FIG. 9 over three fabrication steps (A), (B), and (C), inwhich a molten or liquid state material 11 is blown by a pressureapplied to the other end of the tubing until it forms a semi-sphere. Theextra amount of the molten or liquid state material 13 will be blown outof the tubing, leaving behind a thin balloon-like membrane 12 as shownin (B). The material 12 is then solidified, either (i) thermally using aheater 14, (ii) as the molten material cools down, or (iii) bypolymerization by ultra-violet (UV) exposure if it is a UV-curablepolymer. Liquid rubber material called RTV, which solidifies by itselfover a time period, is another excellent raw material 11 for making thenovel membrane 12 following the procedure shown in FIGS. 9A-C. A lowmelting-temperature glass 11 may be blown into the sphere 12 inside ahigh-melting temperature tubing 2 such as quartz or tungsten. Thespherical nature of the surface 12 does not have to be as pronounced assketched in FIGS. 9A-C. So long as the surface is even slightly convex,it will be much easier to fabricate the membrane following the teachingdescribed here.

In the present invention, the both of fabrication and attachment ofmembranes are accomplished in one step. Thus the fabrication cost willbe substantially lower, especially when the tubing size and the membranethickness are very small. In some applications, the present inventionmay be the only way to make a fiber optic sensor.

It is worthwhile to note that, even though the membrane is shown to beattached at the end of a tubing throughout the invention disclosurehere, it is conceivable that the membrane may be attached onto a holelocated anywhere else, such as on the side wall of a tubing. In otherwords, the teaching may be practiced in embodiments different from theparticular ones shown above, which are provided for the purpose ofillustration.

Another embodiment with a great potential is shown in FIG. 10, in whicha small ball 15 is made separately and then inserted inside the tubing2, or attached to the tubing 2. There are conventional fabricationmethods for making miniature size spheres or balls, and fiber opticsensors can utilize such spheres. In FIG. 10, the surface of the ball 15that is exposed outside the tubing 2 will work as the membrane. In manyapplications, the membrane should be thin. However, there are otherapplications in which the stimulus is very powerful. An example is theengine cylinder pressure sensor. The pressure can reach about 1,000 psi.In such applications, as depicted in FIG. 11, the ball 16 should besolid.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. An optical fiber sensor comprising: a tubing, anoptical fiber residing inside the tubing, and a membrane attached to thetubing; and a plugging material located between said optical fiber andsaid tubing, wherein an air-tight cavity is formed by said pluggingmaterial in combination with said tubing and said optical fiber and saidmembrane, whereby the surface of said membrane becomes resilient due tothe air-cushion effect of said air-tight cavity; wherein the surface ofthe membrane is non-planar, smooth, and taut.
 2. The optical fibersensor of claim 1, wherein the surface of the membrane is convex asviewed from outside of the tubing.
 3. The optical fiber sensor of claim1, wherein the surface of the membrane is spherical.
 4. The opticalfiber sensor of claim 1, wherein the surface of the membrane is conical.5. The optical fiber sensor of claim 1, wherein the surface of themembrane is of a wedge shape.
 6. The optical fiber sensor of claim 1,wherein said membrane is made of a partial surface of a whole sphericalball.
 7. The optical fiber sensor of claim 6, wherein said sphericalball is solid.
 8. The optical fiber sensor of claim 6, wherein saidspherical ball is hollow.